Atom-photon link demoed

By
Eric Smalley,
Technology Research NewsCertain states of atoms and photons can
represent the 1s and 0s of computer information. Atoms are relatively
stable and so are well-suited for storing information, whereas photons
are fleeting but hold onto their information as they travel, which makes
them well-suited for transmitting information.

Practical quantum information processing is likely to require
atoms to process and store information, and photons to transmit information
within and between quantum computers. The trick is finding a way to transfer
information from atoms to photons and back.

Researchers from the University of Michigan have taken a significant
step in that direction by entangling a cadmium ion held in a vacuum by
radio waves, and a single, free-flying ultraviolet photon. An ion is a
charged atom.

Entanglement, dubbed spooky-action-at-a-distance by Einstein,
is a weird ability of particles like atoms and photons. When particles
are entangled, their properties, like polarization, remain linked regardless
of the distance between them. Polarization is the orientation of a photon's
electric field. Entanglement is most often accomplished between like particles.

Entangling an ion and a photon makes it possible to instantly
know the state of the ion by measuring the photon, wherever the photon
is. "Even if the photon traveled [several] light years to Alpha Cantauri
before detection, the Alpha Centaurian who detected the photon would know
what state the ion... on Mother Earth... was in," said Boris Blinov, a
research fellow at the University of Michigan.

This is potentially useful in quantum cryptography, which taps
the properties of particles to provide theoretically perfect security.
Two people can share a series of quantum particles and use them as random
numbers to encrypt and decrypt messages. The process provides perfect
security because when an eavesdropper observes the particles, he unavoidably
alters them, making the security breach detectable.

Ion-photon entanglement also promises to advance quantum computing.
Quantum computers have the potential to solve certain problems like cracking
secret codes and searching large databases far faster than the best possible
classical computer. Quantum computers work by checking every possible
solution to a problem using one set of operations rather than checking
possibilities one by one as today's classical computers do.

The researchers' entanglement method could also be used to teleport
quantum information over long distances and within large quantum computers,
said Blinov. Quantum teleportation works like a fax machine for particles,
with the original destroyed in the process.

The researchers entangled an atom and a photon by trapping a single
cadmium ion in a vacuum using radio-frequency electro-magnetic fields,
then exciting the ion using a 50-nanosecond ultraviolet laser pulse, said
Blinov. A nanosecond is one billionth of a second. "The ion quickly decayed
from this excited state while emitting a single ultraviolet photon," he
said. The researchers detected the ion and measured its polarization.

Before the researchers measured the photon's polarization, the
photon was in a superposition of both possible polarizations -- another
weird quantum trait. The photon's polarization superposition was entangled
with the superposition of the ion's hyperfine levels, which are subtly
different energy levels of the ion's lowest energy, or ground, state.
Once the photon was measured, it assumed a single polarization and at
the same instant the ion assumed a related hyperfine level.

Other experiments have probably produced atom-photon entanglement,
but the entanglement hasn't been directly detected. There are also several
proposals for methods of entangling atoms and photons -- these usually
involve special optical cavities that cause photons to bounce back and
forth many times in a small space containing an atom.

The advantage of the researchers' method is its simplicity. "All
that's required is a trapped ion which can be put into an excited state
and the photon detection system looking at light emitted by the trapped
ion," said Blinov. "The ion excitation is accomplished with conventional
lasers, and emitted photons are collected with conventional lenses," he
said.

Such a simple approach also has a drawback, said Blinov. The entanglement
is probabilistic, meaning it does not result in entanglement every try.
This is because the single photon emitted by the ion is emitted in a random
direction, said Blinov. "We had only a small probability of catching it
with our detectors, covering only about 2 percent of all possible directions
the photons could travel," he said.

The success rate of registering a photon in each experiment was
about one out of 10,000, or about 0.0001 percent, due also to factors
like the imperfect efficiency of the detectors, said Blinov. The researchers
repeated the experiment millions of times to get a large number of successful
trials.

The probability can be increased to as high as a few percent by
optimizing the setup said Blinov. "As long as the success probability
is non-zero, the source of entanglement is still very useful and has many
practical applications," he added.

The researchers' next step is to use the entangled atom and photon
to generate long-distance entanglement of a pair of trapped ions by doubling
the experiment to have a pair of ion traps separated by some distance,
said Blinov. "Ions in both traps will be excited simultaneously, and the
photons transmitted by both ions will be collected and detected," he said.
The right measurements of the photons arriving from the two ions causes
the ions to become entangled. "We can then use the remote entangled ion
pair to implement quantum communication protocols, quantum cryptography,
teleportation and quantum computation," Blinov said.

The scheme could be used for quantum cryptography in five to ten
years, according to Blinov. "Quantum state teleportation and scalable
quantum computation is a more difficult task, but some practical results
may appear in a 10 to 15 year period," he added.

Blinov's research colleagues were David Moehring, Luming Duan
and Christopher Monroe. The work appeared in the March 11, 2004 issue
of Nature. The research was funded by the National Security Agency
(NSA), the Army Research Office (ARO), and the National Science Foundation
(NSF).

Timeline: 5-10 years, 10-15 years Funding: Government TRN Categories: Quantum Computing and Communications; Physics Story Type: News Related Elements: Technical paper, "Observation of Entanglement
between a Single Trapped Atom and a Single Photon," Nature, March 11,
2004